The structure of a water-cooled and water-moderated nuclear reactor of the boiling water type is well known. (See, e.g., U.S. Pat. Nos. 4,548,785 and 5,118,464 to Richardson et al.) As depicted in FIG. 1, a boiling water reactor 2 includes a reactor pressure vessel 4 containing a nuclear reactor core (not shown) submerged in a coolant-moderator such as light water. The core, which is surrounded by an annular shroud 6, includes a plurality of replaceable fuel assemblies (not shown) arranged in spaced relation between an upper core support grid 8 and a lower core support plate 10. A plurality of control rod drive housings (not shown) penetrate the bottom head of the reactor pressure vessel 4 and house control rod drives by which a plurality of control rods (not shown) are selectively insertable among the fuel assemblies for controlling the core reactivity.
Each control rod and the four fuel assemblies comprise a fuel cell of the core. The four fuel assemblies are laterally supported at their upper ends in an opening in the upper core support grid 8 formed by intersecting and interlocking beams. At their lower ends the four fuel assemblies are vertically supported on the fuel assembly support member fitted to the top end of the control rod guide tube, lateral support being provided by passage of the guide tube through an aperture or hole in the lower core support plate 10.
In addition, a plurality of nozzles (only one nozzle 20 of which is shown in FIG. 1) penetrate the bottom head of the reactor pressure vessel. These nozzles are of two types: differential pressure nozzles which provide means for monitoring the differential pressure across the fuel core and liquid poison nozzles which provide means for supplying liquid neutron absorber to the fuel core in the event of a transient overpower event with inability to scram. Each nozzle 20 is supported by an outer tube 12 and extends to the elevation of the upper core support grid 8. The outer tubes 12 are affixed to bottom-head penetrations.
Penetration of the differential pressure and liquid poison nozzles through the bottom head of the reactor pressure vessel 4 is accomplished using stub tubes 14 (see FIG. 7A). Each stub tube, suitably shaped at its bottom end to fit the curvature of the bottom head at its particular location, is secured in a corresponding aperture or hole in the bottom head by a circumferential weld 16. The outer tube 12 is welded to the top end of the stub tube 14 by a circumferential weld 18 after the outer tube 12 is properly positioned vertically.
As is evident from the foregoing, the stub tubes 14 become a part of the pressure vessel boundary and any defect (e.g., cracks) therein can jeopardize the integrity of the pressure system. Under certain conditions, the stub tubes are found to undergo stress corrosion cracking in the heat-affected zone adjacent to the upper weld 18 joining the outer tube and the stub tube. This stress corrosion cracking may result in water leakage from the vessel through the narrow gap between the outer tube 12 and the stub tube 14, an undesirable event necessitating repair.
For the foregoing reasons, the welds which attach the outer tube to the reactor pressure vessel are required to be examined periodically to determine their structural integrity. However, the differential pressure and liquid poison nozzles are inherently difficult to access. Therefore, means for remotely and automatically inspecting the welds by which the differential pressure and liquid poison nozzles are attached to the reactor pressure vessel are needed.